This invention relates to shaft mounting devices and in particular to devices for inhibiting resonant oscillations of a rotatable shaft.
A shaft which extends or projects outward from its mounting bearing, or extends over a considerable distance between bearings has a natural resonant vibrational frequency which depends primarily on the length, mass and stiffness of the shaft in addition to the flexibility of the bearings in which the shaft is mounted. The tendency of a shaft to enter into resonant vibrations is enhanced when the shaft bearings have bearing play; that is, the shaft may be radially vibrated with respect to the structure to which it is mounted by the bearing. When a shaft is excited to vibrate at a frequency which is at or near its natural vibrational frequency, or harmonics of its natural vibrational frequency, the amplitude of shaft vibration will be significantly increased causing unwanted operational noise and in severe cases damage to the bearings.
One prior art technique for suppressing vibrations in a shaft is disclosed in published German patent application No. 1,187,500. That application discloses vibration suppressing intermediate bearings in a curved drive shaft. The curvature of the drive shaft causes a shifting in the first mode resonant frequency of the drive shaft to a range higher than the anticipated revolution frequency of the shaft. Intermediate bearings are provided in the drive shaft, at the point of maximum amplitude of vibration, to suppress vibrations of the shaft in the second mode. The arrangement illustrated in the published application, while effective for the purpose disclosed, is not suitable for suppressing vibrations in an extended shaft, since the diametrically opposed rubber plugs suppress vibrations in only two radial directions.
A similar arrangement, wherein a bearing is mounted by resilient means is disclosed in German patent 1,160,310. The patent shows a bearing which is completely supported by resilient elements. Such a mounting while reducing the transmission of shaft vibrations to the structure on which the shaft is mounted does not alter the natural frequency of resonance of the shaft to reduce the amplitude of vibrations and consequently eliminate the source of vibrational noise and the possibility of excess bearing wear on account of shaft vibration. Also the structure disclosed does not effect the vibrational freedom resulting from bearing play.
Shaft mounting apparatus which overcomes these disadvantages is disclosed in the U.S. patent application Ser. No. 659,114 filed February 18, 1976 of Ulrich Sorgatz and Hans-Jurgen Kogge and assigned to the same assignee as this application. That apparatus includes at least one bearing between the structure and the extended shaft which has bearing play. Also included is a support collar on the structure having an interior surface which is concentric to the shaft. Finally, there is provided a device, permanently resilient in all radial directions, mounted between the shaft and the surface, for shifting the resonant frequency of the shaft to a frequency above the frequency range at which the shaft is excited during operation. As is stated in detail in the aforementioned patent application, the shaft mounting apparatus thus includes one or more bearings having conventional form and a device which centers itself because of its permanent elasticity in all radial directions. Such apparatus is capable of withstanding long periods of operation as may occur, for example, in motor vehicles. The elasticity of the self-centering device is selected in such a way that, by providing additional stiffness to a system which has a tendency to oscillate, the natural frequency of the shaft is moved up to a non-critical frequency range.
It is therefore an object of the present invention to provide shaft mounting apparatus which suppresses shaft vibrations.
It is a further object of the present invention to provide such mounting apparatus which is economically constructed and easily assembled.
It is a further, more particular object of the present invention to provide such mounting apparatus of the type disclosed in the aforementioned patent application which is highly effective and durable despite its simple construction.